The Common Lisp User Interface Environment (CLUE) is a portable system for user interface programming in Common Lisp. CLUE, which is based on the X Window System and the Common Lisp Object System (CLOS), extends the CLX interface to provide an architectural model for the construction of interactive Lisp applications. Modelled on the Xt toolkit library, CLUE could be described as a translation of the Xt “intrinsics” into the domain of Common Lisp and CLOS.